How many naturally occurring elements are found on Earth? Approximately how many synthetic elements have been identified?

91 naturally occurring elements are found on Earth, and at least 21 synthetic elements have been identified.

What was Dmitri Mendeleev's major contribution to the field of chemistry?

Mendeleev's major contribution to chemistry was the system of rows and columns for organizing elements.

Is it possible to distinguish between an element and a compound?

Yes, a compound is made from more than one element.

How are the properties of a compound related to those of the elements that comprise it?

The properties of a compound are different from the properties of the elements it is made of.

A 28 g sample of nitrogen gas combines completely with 6 g of hydrogen gas to form ammonia. What is the mass of the ammonia formed?

The mass of the ammonia formed is 34 grams.

If 68 grams of the substance is present before it is heated, what is the combined mass of the component elements after heating?

The combined mass of the component elements is 68 grams.

A 13 g sample of X combines with a 34 g sample of Y to form the compound XY2. What is the mass of the reactants?

Mass of X: 13 grams

Mass of Y: 17 grams

If 45.98 g of sodium combines with an excess of chlorine gas to form 116.89 g sodium chloride, what mass of chlorine gas is used in the reaction?

70.91 grams of chlorine gas is used in the reaction.

In this reaction, 127 g of copper reacts with 41 g of sulfur. After the reaction is complete, 9 g of sulfur remains unreacted. What is the mass of copper sulfide formed?

The mass of copper sulfide formed is 159 grams.

A 25.3 g sample of an unknown compound contains 0.8 g of oxygen. What is the percent by mass of oxygen in the compound?

The compound is 3.162% oxygen.

If 10.57 g of magnesium reacts completely with 6.96 g of oxygen, what is the percent by mass of oxygen in magnesium oxide?

Magnesium oxide is 39.703% oxygen.

If 28.4 g of mercury oxide decomposes, producing 2 g oxygen, what is the percent by mass of mercury in mercury oxide?

Mercury oxide is 92.958% mercury.

Compound I contains 4.82 g carbon for every 6.44 g of oxygen. Compound II contains 20.13 g carbon for every 53.7 g of oxygen. What is the ratio of carbon to a fixed mass of oxygen for the two compounds?

The ratio of the amount of carbon per fixed mass of oxygen of Compound I to Compound 2 is 2:1.

In this reaction, 123.9 g of phosphorous combines with excess hydrogen to produce 129.9 g of phosphine. After the reaction, 310 g of hydrogen remains unreacted. What mass of hydrogen is used in the reaction? What was the initial mass of hydrogen before the reaction?

6 grams of hydrogen are used.

316 was the initial mass of hydrogen.

A sample of a certain lead compound contains 6.46 grams of lead for each gram of oxygen. A second sample has a mass of 68.54 g and contains 28.76 g of oxygen. Are the two samples the same?